Lumbar interbody fusion is used to treat many diseases of the lumbar spine, such as spinal stenosis, spondylolisthesis, and spinal deformation.1 Among the various fusion methods, posterior approaches are most commonly used, including conventional posterior lumbar interbody fusion (PLIF)234 and transforaminal lumbar interbody fusion (TLIF), first introduced by Harms et al.5

Unlike conventional PLIF, TLIF can achieve decompression and interbody arthrodesis by preserving the opposite facet and lamina in a unilateral approach, while reducing the amount of soft-tissue injury as well as the extent of thecal-sac retraction.678 Nevertheless, both muscle atrophy due to the extensive skin incisions and perivertebral muscle detachment are common postoperative problems.6910 Recently, minimally invasive techniques using a tubular retractor have been shown to preserve much of the lumbar musculature while splitting the muscle to greatly reduce intraoperative bleeding.111213 Among the advantages are a reduction in pain immediately after surgery, a faster recovery, and a shorter hospital stay and thus a faster return to daily life.14151617

Many studies have reported that minimally invasive TLIF (MI-TLIF) provides satisfactory treatment comparable to other fusion methods,12181920 with outcomes similar to those obtained with the conventional fusion method in terms of interbody fusion, complications, and clinical results. However, in MI-TLIF annulectomy and disc resection are performed on only 1 side and only a single cage is inserted, such that the area of disc space preparation may be smaller than in a conventional bilateral PLIF. Furthermore, since the laminar resection is unilateral, much less autogenous local bone can be harvested and an allograft is thus additionally required. These issues have raised concern as to whether, over the long term, the fusion rate is lower than that achieved with conventional PLIF. Nonetheless, an advantage of MI-TLIF is that it minimizes the impact on the connection between the index level and the adjacent segment by preserving the surrounding structures and the interconnecting soft tissues, especially on the contralateral side. Accordingly, compared to conventional open interbody fusion, MI-TLIF should be less likely to cause adjacent segment degeneration (ASD). However, long-term follow-up studies are lacking as most reports have a maximum follow-up of 5 years.

Here we report the clinical and radiological results obtained in a 10-year follow-up of patients with degenerative lumbar disease treated by MI-TLIF.

Thirty patients underwent MI-TLIF and were followed-up for at least 10 years. The mean duration of follow-up was 11.1 years (range, 10.0–12.7 years). The mean age at surgery was 60.1 ± 11.3 years (range, 40–80 years). The number of fused levels was 1 segment in 25 patients (83.3%), and 2 segments in 5 patients (16.7%). L4–5 was the most frequently operated level, followed by L5–S1 and L3–4. The main pathological indications for surgery were spinal stenosis (n = 14, 46.7%), spondylolisthesis (n = 13, 43.3%), recurrent disc herniation (n = 2, 6.7%), and internal disc disruption (n = 1, 3.3%). The baseline characteristics of the patients are listed in Table 1.

As life expectancy in many countries continues to increase, due to advances in medical technology, the demand for effective spinal surgery is also likely to increase. Previously, most studies examined perioperative complications, but the focus of recent research has shifted to a consideration of the long-term results, especially long-term clinical outcomes and the inevitable occurrence of ASD following fusion surgery. The goal of minimally invasive techniques is to preserve the paraspinal muscles and soft tissues and thus maintain spinal stability between the index and adjacent segments. For this reason, MI-TLIF has been widely adopted and its results are comparable to those achieved with conventional PLIF or open TLIF. However, most of the studied patients have been followed-up for < 5 years.

In our long-term study, while the union rate 1 year after MI-TLIF was relatively low (77%), it increased to 91% at 5 years and 94% at 10 years, thus ultimately resulting in a high rate of union (Fig. 5). A limitation of MI-TLIF compared to conventional open PLIF is that the volume of local bone available as a graft is limited, because the procedure includes only a hemilaminectomy, performed through a tubular retractor. Although an allo-chip bone graft and DBM are used to compensate for this deficit, there is concern that it will lead to a low rate of union. Bone morphogenic protein (BMP) was not used in any of our patients. Nonetheless, our study was able to demonstrate the reliability of MI-TLIF as a fusion method as well as successful spinal fusion even without BMP, with results comparable to those reported in previous studies. For example, in a study that included MI-TLIF patients with a minimum 5-year follow-up, the overall fusion rate was 97.7%, with complete fusion and interbody bony bridging achieved in 84.1%.26 In another study of MI-TLIF in which patients were followed-up for 5 years,27 solid fusion was achieved in 72% of the patients by the second postoperative year and in 74% by the fifth postoperative year. Bin Abd Razak et al.28 performed 1-level MI-TLIF in patients with spondylolisthesis and reported grade 1 fusion 6 months, 2 years, and 5 years postoperatively in 39.3%, 92.9%, and 96.4%, respectively. However, in their study, in 15 patients the bone graft included additional autogenous bone from the posterior iliac crest, and in 41 patients a larger amount of DBM was used than in our study.

Several studies have described ASD after lumbar fusion surgery, but the incidence varied depending on the definition of ASD. The reported incidence of R-ASDeg is 31–82.6%293031 whereas the incidence of O-ASD is much lower, 4.9–24%.3233 The incidence of ASD also varies because it depends on the duration of follow-up. In our study, at the 10-year follow-up exam the incidence of R-ASDeg was 50% and that of O-ASD 23%. When these results are compared with those of previous studies of open PLIF, in which the patients were followed-up for at least 10 years, the incidence of R-ASDeg was lower in our study. Okuda et al.25 reported that R-ASDeg developed in 75% and O-ASD in 15% of patients with degenerative spondylolisthesis who underwent open PLIF and were followed-up for > 10 years. In a similar study, Nakashima et al.34 reported R-ASDeg in 68% and O-ASD in 9.9%. The lower incidence of O-ASD than in our study may have been due to differences in the eligibility criteria for reoperation. Further prospective studies are needed to determine whether the 2 surgical methods result in different rates of ASD.

The R-ASDeg observed in our study tended to involve the proximal adjacent segments more often than the distal adjacent segments, as also described by other investigators. In Bydon et al.,35 disc degeneration and reoperation in patients with ASD who underwent L4–5 discectomy were seen more often in the proximal than in the distal adjacent segments. Another study reported that 24 patients who underwent L4–5 fusion surgery required reoperation due to ASD of the proximal adjacent segment, L3–4, compared to 3 patients reoperated for ASD of the distal adjacent segment.36 Among our patients, L4–5 was the most frequently operated site. The distal adjacent segment, L5–S1, is supported by the tightly connected lumbosacral ligaments, which limits abnormal movement, and by the sacrum and pelvis, which increase biomechanical stability.43738

However, the implications of R-ASDeg are controversial, with at least 2 studies failing to find an association of R-ASDeg with clinical symptoms.2139 While the incidence of R-ASDeg and O-ASD differ significantly, our patients with R-ASDeg suffered statistically worse leg pain, as evaluated using the VAS and ODI. This result suggests that R-ASDeg is related to the deterioration of clinical symptoms and is a pre-stage to O-ASD progression.

Several studies have reported long-term clinical results after MI-TLIF. In the series of Bin Abd Razak et al.,28 the improvements in the VAS for back and leg pain and in the ODI, 6 months after surgery, were maintained for up to 5 years; similar results were obtained with the Short-Form 36 health survey and Neurogenic Symptoms Score. In the study of Rouben et al.,40 improvements in the ODI and in the VAS of back pain, determined 3 months postoperatively, were maintained at 49 months, regardless of the preoperative diagnosis. In our study, the clinical outcomes after MI-TLIF were maintained for a longer period of time than in previous studies; in our patients the improvements in back pain and leg pain in the first postoperative year were maintained 10 years after surgery. In other MI-TLIF studies the follow-up time was limited to 5 years, whereas our study shows that MI-TLIF continues to provide good clinical results even 10 years after the procedure.

The analyses of our MI-TLIF patients, classified according to the fusion level length, preoperative diagnosis, and age showed that, similar to other studies, these subgroups did not differ in terms of the postoperative improvements in back pain and leg pain on VAS and ODI, either after surgery or at the 10-year follow-up. Although a previous study in which patients were assessed using the VAS and ODI reported greater improvement in those with multi-level than with 1-level fusion, the preoperative VAS and ODI were higher in the 1-level group whereas the postoperative values were similar to those of the latter; thus, the trend over time was in agreement with our findings.40 In another study, patients treated with 1-level MI-TLIF were classified into isthmic spondylolisthesis and degenerative spondylolisthesis subgroups based on the preoperative diagnosis. Outcomes were evaluated using the VAS and ODI as well as patient satisfaction and return to work rates, but none of the differences between the subgroups were significant.26 These results together with those of other studies demonstrate the good long-term clinical outcomes that can be achieved with MI-TLIF, regardless of the preoperative diagnosis, patient age, or the need for 2-stage fusion.

Failure to follow-up in long-term follow-up studies can lead to misinterpretation and biased conclusions. The possibility that follow-up rates will not be the same between patients with good and bad clinical outcomes is always a difficult question to answer and a common limitation of human studies. Interestingly, in a recent study comparing the clinical outcomes of patients with and without postoperative follow-up for lumbar degenerative disease, there were no significant differences between groups.41 In this study, to minimize this limitation, the survival rate was analyzed by dividing the group with less than 10 years of follow-up and the group with more than 10 years of follow-up using the Kaplan-Meier method. When survival analysis was conducted using R-ASDeg and O-ASD as events, there was no significant difference in the survival rate between groups according to follow-up period. Therefore, although the follow-up rate of this study is not high, it is expected to provide meaningful results and will be a valuable reference for future randomized prospective studies.

A strength of our study was that it was performed at a single center and all of the operations were performed by a single senior spine surgeon (Jae Chul Lee). These features increased the reliability of the study, by reducing the number of variables. Another advantage of our study was a follow-up of > 10 years after MI-TLIF. However there were also limitations that should be addressed. These included the retrospective design, which may have resulted in observer bias, a lack of standardization of the data, and a lower quality than prospective studies, as well as the relatively small population. This resulted in a lack of statistical power in some of the analyses. Despite these weaknesses, our study is the only 1 thus far to present 10-year follow-up data for MI-TLIF. Studies of patients followed prospectively for at least 10 years will provide further insights into the clinical advantages of MI-TLIF.

In conclusion, in patients with lumbar spine disease treated by MI-TLIF, the improved clinical and radiological outcomes obtained postoperatively were maintained for up to 10 years and the fusion rates at the end of follow-up were high. However, during the first 10 postoperative years, 50% of patients developed R-ASDeg, which was also characterized by worse leg pain as assessed using the VAS and ODI tools.